Theory of mind is the ability to think about the mental states, such as thoughts, beliefs, and intentions, of oneself and others. It is critical for interacting with and learning from other people; individuals with developmental conditions associated with impaired theory of mind like autism show atypical developmental outcomes. Despite the fundamental role of theory of mind in human social and cognitive function, when and how it develops remains unclear. On the one hand, it is not until 3-5 years of age that children are able to successfully answer explicit questions about the mental states of others. On the other hand, looking behavior in infants as young as 6-7 months suggests that we track the mental states of others well before we are able to answer explicit questions about them. The main question that has been debated over the last decade in the scientific literature is how spontaneous social-cognitive sensitivities to the mental states of others apparent through looking behavior of infants relate to later developing explicit theory of mind reasoning. The fundamental barrier has been reconciling results from different measures and methodologies in infants and children (and adults). Here we propose to further pursue a new, promising neuroscience methodology to measure theory of mind development throughout the lifespan and begin to validate it against known, relevant behavioral and brain metrics. We will combine video stimuli modeled after behavioral looking time studies and that only require passive viewing with an optical brain measure called near-infrared spectroscopy (NIRS) that can be used with young infants, children, or adults. First, we will attempt to develop robust metrics of functional brain sensitivity and selectivity for theory of mind using existing NIRS data from 6-10-month-old infants and new data from 4-5-year-old children. Next, we will begin to validate those metrics by analyzing their relationship to behavioral abilities in theory of mind in a cross-sectional group of 4-5-year-old children. Finally, we will pilot the feasibility of studying theory of mind development in brain and behavior longitudinally by directly linking previously collected infant NIRS data to new data collected on the same individuals at 4-5 years. The way forward we are presenting, if successful, will overcome current measurement limitations in the field and provide empirical justification to request support (e.g., R01) for a full-scale study of theory of mind development using our novel approach. This has the potential to resolve longstanding questions regarding theory of mind development by directly testing the relationship between spontaneous theory of mind processing in infancy and explicit theory of mind reasoning in childhood. In the future, it may also help us better understand developmental disabilities characterized by impaired social cognition like autism, where early detection and high-quality measurement are very challenging. The unprecedented nature of the brain and behavioral data we propose to collect also has the potential bridge broader theories on and applications of understanding the relationship between brain and cognition over early development as well.